Nanowire Dispersion Compositions and Uses Thereof

ABSTRACT

Nanowire dispersion compositions (and uses thereof) are disclosed comprising a plurality of inorganic nanowires suspended in an aqueous or non-aqueous solution comprising at least one low molecular weight and/or low HLB (Hydrophile-Lipophile Balance) value dispersant. Methods of further improving the dispersability of a plurality of inorganic nanowires in an aqueous or non-aqueous solution comprise, for example, oxidizing the surface of the nanowires prior to dispersing the nanowires in the aqueous or non-aqueous solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/399,307, filed Apr. 6, 2006, which claims the benefit of U.S.Provisional Patent Application No. 60/671,131, filed Apr. 13, 2005,which is incorporated in its entirety herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable

BACKGROUND OF THE INVENTION

Nanotechnology has been simultaneously heralded as the nexttechnological evolution that will pave the way for the next societalevolution, and lambasted as merely the latest batch of snake oil peddledby the technically overzealous. Fundamentally both sides of the argumenthave a number of valid points to support their position. For example, itis absolutely clear that nanomaterials possess very unique and highlydesirable properties in terms of their chemical, structural andelectrical capabilities. However, it is also clear that, to date, therehas been very little discussion of technology for manufacturing andintegrating nanoscale materials into the macroscale world in areasonable commercial fashion and/or how to assemble these nanomaterialsinto more complex systems for the more complex prospective applications,e.g., nanocomputers, nanoscale machines, electronic devices etc. Avariety of early researchers have proposed a number of different ways toaddress the integration and assembly questions by waiving their handsand speaking of molecular self assembly, electromagnetic assemblytechniques and the like. However, there has been either little publishedsuccess or little published effort in these areas.

In certain cases, uses of nanomaterials have been proposed that exploitthe unique and interesting properties of these materials more as a bulkmaterial than as individual elements requiring individual assembly. Forexample, Duan et al., Nature 425:274-278 (September 2003), describes ananowire based transistor for use in large area electronic substrates,e.g., for displays, antennas, etc., that employs a bulk processed,oriented semiconductor nanowire film or layer in place of a rigidsemiconductor wafer. The result is an electronic substrate that performson par with a single crystal wafer substrate, but that is manufacturableusing conventional and less expensive processes that are used in thepoorer performing amorphous semiconductor processes, and is moreamenable to varied architectures, e.g., flexible and/or shapedmaterials. In accordance with this technology, the only new processrequirement is the ability to provide a film of nanowires that aresubstantially oriented along a given axis. The technology for suchorientation has already been described in detail in, e.g., InternationalPublication Nos. WO 03/085700, WO 03/085701 and WO 2004/032191, as wellas U.S. Pat. No. 7,151,209 (the full disclosures of each of which arehereby incorporated by reference herein, in their entirety for allpurposes) and is readily scalable to manufacturing processes.

In another exemplary case, bulk processed nanocrystals have beendescribed for use as a flexible and efficient active layer forphotoelectric devices. In particular, the ability to provide a quantumconfined semiconductor crystal in a hole conducting matrix (to providetype-II bandgap offset), allows the production of a photoactive layerthat can be exploited either as a photovoltaic device or photoelectricdetector. When disposed in an active composite, these nanomaterials aresimply processed using standard film coating processes that areavailable in the industry. See, e.g., U.S. Pat. No. 6,878,871incorporated herein by reference in its entirety for all purposes.

Regardless of the applications to which nanomaterials are to be put,there exists a need for the improved production, processing andintegration of these materials into their ultimate application ordevice. In particular, one of the remaining challenges in therealization of functional nanostructures is to figure out ways to gainreliable control over their surface chemistry and to improve theirdispersability in aqueous and non-aqueous media and the like to improvethe handling and processability of these materials. The presentinvention meets these and a variety of other needs.

BRIEF SUMMARY OF THE INVENTION

The present invention is generally directed to nanowire dispersioncompositions for dispersing nanowires (e.g., inorganic nanowires such assemiconductor nanowires) in different aqueous or non-aqueous solvents(e.g., distilled water, ethanol, methanol, dimethylformamide (DMF),combinations thereof, etc.) using one or more surfactants to improve thesolubility and processability of the resultant nanowire thin films.Dispersing of nanowires in different compositions facilitates themanipulation of the wires and provides stable films of nanowires whichare useful building blocks for high performance electronics and otherdevices and systems described further below. The nanowire dispersioncomposition can be formulated to exhibit a broad range of desiredproperties such as surface tension and viscosity, to minimize wireaggregation, to exhibit rapid drying when applied as a thin film to adevice substrate, and to provide a dispersion composition that is stableover useful periods of time.

In accordance with a first aspect of the present invention, a nanowiredispersion composition is disclosed which generally comprises aplurality of nanowires suspended in a solvent (e.g., ethanol, water,methanol, etc. and/or combinations thereof) and comprising at least onelow molecular weight (e.g., less than about 10,000, e.g., less thanabout 5,000, e.g., less than about 3,000) surfactant selected from thegroup comprising an anionic, cationic, nonionic, amphoteric or polymericsurfactant. In one presently preferred embodiment, the nanowiredispersion composition comprises a dispersant selected from a nonionicor polymeric dispersant. For example, the dispersant is selected fromthe group comprising SilWet®, Surfynol®, a Pluronic polymer, andcombinations and mixtures thereof. The combination of the dispersantwith one or more low chain polymeric dispersants such as Ammoniumpolyacrylate Acrysol G-111 (AG-111), polyvinyl alcohol (PVA), andpolyvinyl pyrrolidone (PVP) has further been found to provide enhancedwire dispersity and coating uniformity.

It has also been found that dispersants with both lower molecularweights and lower HLB (Hydrophile-Lipophile Balance) values (e.g., lessthan about 20, e.g., less than about 15, e.g., less than about 10)provide the best wire dispersion and coating uniformity. The nanowiredispersion composition may comprise, for example, between about 0.01 to20% by weight of dispersant, for example, between about 1 to 15 wt %dispersant, for example, between about 1 to 8 wt % dispersant. Thenanowire dispersion composition may also comprise one or more binders,e.g., one or more water soluble or dissipatable polymers selected fromthe group comprising polyvinyl alcohol, polyvinyl pyrrolidone, methylcellulose, and hydroxylethyl cellulose and the like. The dispersantcompositions of the present invention find particular utility withinorganic nanowires such as semiconductor (e.g., silicon) or metal oxidenanowires, but can also be used with other nanowire materials as wellsuch as conductive polymers, ceramics etc. The nanowire dispersioncompositions of the present invention work particularly well withnanowires having an aspect ratio greater than about 10, e.g., greaterthan about 100, e.g., greater than about 1000.

Optionally, in another aspect of the present invention, the nanowiresmay be oxidized prior to being suspended in the nanowire dispersioncompositions of the present invention, e.g., the nanowires may bethermally oxidized in a rapid thermal oxidation (RTO) chamber or in aconventional oxidation furnace, in order to further improve the coatingand solution uniformity of the nanowire dispersion composition.

The present invention provides flexible (e.g., plastic) or rigid (e.g.,glass or semiconductor) substrates comprising the nanowire dispersioncomposition of the present invention deposited as a thin film thereon,which substrates can be used in various electrical systems, opticalsystems, consumer electronics, industrial electronics, wireless systems,space applications, or any other application including, for example,active matrix displays, antenna arrays, radiofrequency identificationtags, smart cards, optical sensors, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a single crystal semiconductor nanowire core(hereafter “nanowire”) 100.

FIG. 1B shows a nanowire 110 doped according to a core-shell structure.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW) (and nanorod, nanotube, andnanoribbon etc.) technologies and other functional aspects of thesystems (and components of the individual operating components of thesystems) may not be described in detail herein. Furthermore, forpurposes of brevity, the invention is frequently described herein aspertaining to nanowires.

Individual inorganic nanowires (NWs) (e.g., semiconductor nanowires suchas silicon nanowires) can be used to fabricate nanoscale field effecttransistors (FETs) with electronic performance comparable to and in somecase exceeding that of the highest-quality single-crystal materialscurrently commercially available. These nano-FETs are extending Moore'slaw toward the molecular level. They are, however, currently difficultto implement for production-scale nanoelectronics due to the complexityand limited scalability of the device fabrication processes. Successfulimplementation of nanowires in electronic device requires methods todeposit and pattern them over wide area substrates uniformly at highdensity and with minimum aggregation. To do so requires the use ofdispersants or additives to properly disperse the nanowires in solutionso that they can be deposited on electronic device substrates in acontrolled and reproducible manner suitable for commercial applications.

It should be appreciated that although nanowires are frequently referredto, the techniques described herein are also applicable to othernanostructures, such as nanorods, nanotubes, nanotetrapods, nanoribbonsand/or combinations thereof. It should further be appreciated that theprocessing techniques described herein could be used with anysemiconductor device type, and other electronic component types.Further, the techniques would be suitable for application in electricalsystems, optical systems, consumer electronics, industrial electronics,wireless systems, space applications, or any other application.

As used herein, an “aspect ratio” is the length of a first axis of ananowire divided by the average of the lengths of the second and thirdaxes of the nanowire, where the second and third axes are the two axeswhose lengths are most nearly equal to each other. For example, theaspect ratio for a perfect wire would be the length of its long axisdivided by the diameter of a cross-section perpendicular to (normal to)the long axis.

The term “heterostructure” when used with reference to nanowires refersto structures characterized by at least two different and/ordistinguishable material types. Typically, one region of the nanowirecomprises a first material type, while a second region of the nanowirecomprises a second material type. In certain embodiments, the nanowirecomprises a core of a first material and at least one shell of a second(or third etc.) material, where the different material types aredistributed radially about the long axis of a nanowire, a long axis ofan arm of a branched nanocrystal, or the center of a nanocrystal, forexample. A shell need not completely cover the adjacent materials to beconsidered a shell or for the nanowire to be considered aheterostructure. For example, a nanowire characterized by a core of onematerial covered with small islands of a second material is aheterostructure. In other embodiments, the different material types aredistributed at different locations within the nanowire. For example,material types can be distributed along the major (long) axis of ananowire. Different regions within a heterostructure can compriseentirely different materials, or the different regions can comprise abase material.

As used herein, a “nanostructure” is a structure having at least oneregion or characteristic dimension with a dimension of less than about500 nm, e.g., less than about 200 nm, less than about 100 nm, less thanabout 50 nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, for example, substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, or acombination thereof. In one aspect, each of the three dimensions of thenanostructure has a dimension of less than about 500 nm, for example,less than about 200 nm, less than about 100 nm, less than about 50 nm,or even less than about 20 nm.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and preferably, less than 100 nm, and has an aspectratio (length:width) of greater than 10, preferably greater than 50, andmore preferably, greater than 100, or greater than 1000.

The nanowires of this invention can be substantially homogeneous inmaterial properties, or in certain embodiments can be heterogeneous(e.g., nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, or amorphous. Nanowires can have a variable diameter orcan have a substantially uniform diameter, that is, a diameter thatshows a variance less than about 20% (e.g., less than about 10%, lessthan about 5%, or less than about 1%) over the region of greatestvariability and over a linear dimension of at least 5 nm (e.g., at least10 nm, at least 20 nm, or at least 50 nm). Typically the diameter isevaluated away from the ends of the nanowire (e.g. over the central 20%,40%, 50%, or 80% of the nanowire). A nanowire can be straight or can bee.g. curved or bent, over the entire length of its long axis or aportion thereof. In certain embodiments, a nanowire or a portion thereofcan exhibit two- or three-dimensional quantum confinement. Nanowiresaccording to this invention can expressly exclude carbon nanotubes, and,in certain embodiments, exclude “whiskers” or “nanowhiskers”,particularly whiskers having a diameter greater than 100 nm, or greaterthan about 200 nm. Examples of such nanowires include semiconductornanowires as described in Published International Patent ApplicationNos. WO 02/17362, WO 02/48701, and WO 01/03208, carbon nanotubes, andother elongated conductive or semiconductive structures of likedimensions, which are incorporated herein by reference.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis so that the couplednanorods span all the way between electrodes. Alternatively, two or morenanorods can be substantially aligned along their longitudinal axis, butnot coupled together, such that a small gap exists between the ends ofthe two or more nanorods. In this case, electrons can flow from onenanorod to another by hopping from one nanorod to another to traversethe small gap. The two or more nanorods can be substantially aligned,such that they form a path by which electrons can travel betweenelectrodes.

A wide range of types of materials for nanowires, nanorods, nanotubesand nanoribbons can be used, including semiconductor material selectedfrom, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, B—C,B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

The nanowires can also be formed from other materials such as metalssuch as gold, nickel, palladium, iradium, cobalt, chromium, aluminum,titanium, ruthenium, tin and the like, metal alloys, polymers,conductive polymers, ceramics, and/or combinations thereof. Other nowknown or later developed conducting or semiconductor materials can beemployed.

In certain aspects, the nanowire may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si.; or an n-type dopant selected from a group consisting of: Si,Ge, Sn, S, Se and Te. Other now known or later developed dopantmaterials can be employed.

Additionally, the nanowires or nanoribbons can include carbon nanotubes,or nanotubes formed of conductive or semiconductive organic polymermaterials, (e.g., pentacene and transition metal oxides).

Hence, although the term “nanowire” is referred to throughout thedescription herein for illustrative purposes, it is intended that thedescription herein also encompass the use of nanotubes (e.g.,nanowire-like structures having a hollow tube formed axiallytherethrough). Nanotubes can be formed in combinations/thin films ofnanotubes as is described herein for nanowires, alone or in combinationwith nanowires, to provide the properties and advantages describedherein.

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom”, etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

Types of Nanowires and their Synthesis

FIG. 1A illustrates a single crystal semiconductor nanowire core(hereafter “nanowire”) 100. FIG. 1A shows a nanowire 100 that is auniformly doped single crystal nanowire. Such single crystal nanowirescan be doped into either p- or n-type semiconductors in a fairlycontrolled way. Doped nanowires such as nanowire 100 exhibit improvedelectronic properties. For instance, such nanowires can be doped to havecarrier mobility levels comparable to bulk single crystal materials.

FIG. 1B shows a nanowire 110 doped according to a core-shell structure.As shown in FIG. 1B, nanowire 110 has a doped surface layer 112, whichcan have varying thickness levels, including being only a molecularmonolayer on the surface of nanowire 110.

The valence band of the insulating shell can be lower than the valenceband of the core for p-type doped wires, or the conduction band of theshell can be higher than the core for n-type doped wires. Generally, thecore nanostructure can be made from any metallic or semiconductormaterial, and the shell can be made from the same or a differentmaterial. For example, the first core material can comprise a firstsemiconductor selected from the group consisting of: a Group II-VIsemiconductor, a Group III-V semiconductor, a Group IV semiconductor,and an alloy thereof. Similarly, the second material of the shell cancomprise a second semiconductor, the same as or different from the firstsemiconductor, e.g., selected from the group consisting of: a GroupII-VI semiconductor, a Group III-V semiconductor, a Group IVsemiconductor, and an alloy thereof. Example semiconductors include, butare not limited to, CdSe, CdTe, InP, InAs, CdS, ZnS, ZnSe, ZnTe, HgTe,GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, and PbTe. Asnoted above, metallic materials such as gold, chromium, tin, nickel,aluminum etc. and alloys thereof can be used as the core material, andthe metallic core can be overcoated with an appropriate shell materialsuch as silicon dioxide, carbides, nitrides or other insulatingmaterials

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape Control of CdSeNanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals”; U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers”; U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same”; and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process”; U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-,Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys”;U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques”; and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Furthermore, nanowire structures with one or multiple shells can also befabricated. Synthesis of core-shell nanowire (and other nanocrystal)heterostructures are described in, e.g., Berkeley U.S. Pat. No.6,996,147; co-assigned and pending U.S. Ser. No. 60/605,454 entitled“Processes for Manufacturing, Harvesting, and Integrating Nanowires intoFunctional Nanowire Based Devices,” filed Aug. 30, 2004; Peng et al.(1997) “Epitaxial growth of highly luminescent CdSe/CdS core/shellnanocrystals with photostability and electronic accessibility” J. Am.Chem. Soc. 119, 7019-7029; Dabbousi et al. (1997) “(CdSe)ZnS core-shellquantum dots: Synthesis and characterization of a size series of highlyluminescent nanocrysallites” J. Phys. Chem. B 101, 9463-9475; Manna etal. (2002) “Epitaxial growth and photochemical annealing of gradedCdS/ZnS shells on colloidal CdSe nanorods” J. Am. Chem. Soc. 124,7136-7145, the entire contents of each of which are incorporated byreference herein. Similar approaches can be applied to the growth ofother core-shell nanostructures including nanowires.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and US Publication No. 20040026684.Similar approaches can be applied to growth of other heterostructures.

In one embodiment of the invention, the nanowires may be synthesized ona growth substrate, and then transferred and incorporated into theultimate device substrate using the nanowire dispersion compositions ofthe present invention. For example, in certain aspects, inorganicsemiconductor or semiconductor oxide nanowires are grown on the surfaceof a growth substrate using a colloidal catalyst basedvapor-liquid-solid (VLS) synthesis method. In accordance with thissynthesis technique, a colloidal catalyst (e.g., gold, platinum etc.particles) is deposited upon the desired surface of the substrate. Thesubstrate including the colloidal catalyst is then subjected to thesynthesis process which generates nanowires attached to the surface ofthe substrate. Other synthetic methods include the use of thin catalystfilms, e.g., 50 nm or less, deposited over the surface of the substrate.The heat of the VLS process then melts the film to form small dropletsof catalyst that forms the nanowires. Typically, this latter method maybe employed where fiber diameter homogeneity is less critical to theultimate application. Typically, catalysts comprise metals, e.g., goldor platinum, and may be electroplated or evaporated onto the surface ofthe substrate or deposited in any of a number of other well known metaldeposition techniques, e.g., sputtering etc. In the case of colloiddeposition the colloids are typically deposited by first treating thesurface of the substrate so that the colloids adhere to the surface. Thesubstrate with the treated surface is then immersed in a suspension ofcolloid. Furthermore, as described above, nanowire structures with oneor multiple shells can also be fabricated.

Following growth of the nanowires, the nanowires are then harvested fromtheir synthesis location. As described further below, the nanowirestypically are introduced into a solution having one or more surfactantsand/or other compounds therein. A variety of other deposition methods,e.g., as described in U.S. Pat. No. 7,067,328, and U.S. Pat. No.6,962,823, the full disclosures of which are incorporated herein byreference in their entirety for all purposes, may also be used todeposit nanowires onto a device substrate.

Nanowire Dispersion Compositions

Nanowires may be suspended in various anionic, nonionic, amphoteric andcationic surfactants and polymers to improve their dispersability inliquid media such as aqueous and polar solvents. Preferably the nanowiresolution comprises at least one dispersant. Suitable dispersants includeboth polymeric surfactants and anionic, cationic, amphoteric andnonionic surfactants as described further below.

In particular, the nanowire dispersion compositions of the inventioninclude a liquid medium or solution having dispersed therein a pluralityof nanowires (e.g., inorganic nanowires such as semiconductor nanowires,e.g., silicon nanowires). The liquid medium can be entirely water, cancontain water in combination with one or more organic solvents, or canbe entirely organic solvent. In one embodiment, the liquid mediumcontains water, e.g., at least 20% by weight water, for example, betweenabout 50% and 100% water.

An organic solvent can be included in the liquid medium, e.g., tocontrol surface tension of the nanowire dispersion composition, to allowdissolution of the dispersant, or, as a minor component of any of theingredients, e.g., an organic solvent may be present in a surfactantadded as an ingredient to the dispersion composition.

The organic solvent, when present, can be any number of organic solventsknown to those of ordinary skill in the art. For example, suitablewater-miscible organic solvents include C₁₋₅-alkanols, e.g. methanol,ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanoland isobutanol; amides, e.g. dimethylformamide and dimethylacetamide;ketones and ketone alcohols, e.g. acetone and diacetone alcohol;C₂₋₄-ether, e.g. tetrahydrofuran and dioxane; alkylene glycols orthioglycols containing a C₂₋₆ alkylene group, e.g. ethylene glycol,propylene glycol, butylene glycol, pentylene glycol and hexylene glycol;poly(alkylene-glycol)s and thioglycol)s, e.g. diethylene glycol,thiodiglycol, polyethylene glycol and polypropylene glycol; polyols,e.g. glycerol and 1,2,6-hexanetriol; and lower alkyl glycol andpolyglycol ethers, e.g., 2-methoxyethanol, 2-(2-methoxyethoxy)ethanol,2-(2-ethoxyethoxy) ethanol, 2-(2-butoxyethoxy)ethanol,3-butoxypropan-1-ol, 2-[2-(2-methoxyethoxy)-et-hoxy]ethanol,2-[2-(2-ethoxyethoxy)ethoxy]-ethanol; cyclic esters and cyclic amides,e.g. optionally substituted pyrrolidones; sulpholane; and mixturescontaining two or more of the aforementioned water-miscible organicsolvents.

The amount of organic solvent and/or water within the liquid medium candepend on a number of factors, such as the particularly desiredproperties of the nanowire dispersion composition such as the viscosity,surface tension, drying rate, etc., which can in turn depend on factorssuch as the type of substrate the nanowire dispersion is intended to bedeposited on, the size, shape, composition, and/or aspect ratio of thenanowires, etc. For some applications, the liquid medium can comprise anaqueous solution including a mixture of water and solvent. For suchwater-based media, preferred amounts of organic solvent can be in therange from 0 to about 80 parts by weight organic solvent based on 100parts by weight of the liquid medium (defined as water plus organicsolvent), for example, from about 2 to 50 parts by weight organicsolvent per 100 parts liquid medium.

To achieve a particularly good dispersion additional dispersants may beadded. Preferably the dispersant is a surfactant. Exemplary surfactantsare anionic surfactants such as; sodium dodecyl sulphate, ammoniumdodecyl benzene sulphonate, sodium nonoxynyl phosphate, sodium dioctylsulphosuccinate; nonionic surfactants such as alkyl phenol ethoxylates,alkyl ethoxylates, polyoxyethylene polyoxypropylene block copolymers;cationic surfactants such as; dodecyl trimethyl ammonium bromide,bis(2-hydroxyethyl)tallow amine; and amphoteric surfactants such asammonium cocoaminopropionamide. The choice of dispersants will begoverned by many factors including; the amount of polymer to bedispersed; the nature of the liquid medium; the size, shape,composition, or aspect ratio of the nanowires; the material type ofnanowires, etc. For an optimum dispersion it may be necessary to use amixture of several dispersants.

Specific examples of commercially available anionic surfactants that canbe used in practicing the present invention include, for example, sodiumdodecylbenzenesulfonate (SDBS—Sigma Aldrich, Inc.); sodium alkyl allylsulfosuccinate (TREM—Cognis Corporation); sodium n-lauroylsarcosinate(Sarkosyl—Sigma Aldrich); sodium dodecyl sulfate (SDS—Sigma Aldrich);polystyrene sulfonate (PSS—Sigma Aldrich). Specific examples ofcommercially available cationic surfactants that can be used inpracticing the methods of the present invention include, for example,dodecyltrimethylammonium bromide (DTAB—Sigma Aldrich) andcetyltrimethylammonium bromide (CTAB—Sigma Aldrich). Specific Examplesof commercially available nonionic surfactants include for exampleSurfynol 420, Surfynol 440, Surfynol 211, and Surfynol 221 (Airproducts, PA); Brij (Sigma Aldrich); Tween (Sigma Aldrich); Triton X(e.g., Triton X-100, Triton X-405, etc.—Sigma Aldrich); Span (e.g., Span20, Span 40, Span 80, Span 85, etc.—Sigma Aldrich)poly(vinylpyrrolidone) (PVP—Sigma Aldrich); Silwet (e.g., Silwet® L-77,L-7608, and L-7280—General Electric); PEO-PBO-PEO triblock copolymer(EBE—Dow Chemical Corp.); PEO-PPO-PEO triblock polymer (Pluronic seriesof compounds—BASF).

It was found by the inventors of the present invention that surfactantswith lower molecular weight (e.g., those having a molecular weight lessthan about 10,000, e.g. less than about 5,000, e.g., less than about3,000) and/or lower HLB (Hydrophile-Lipophile Balance) values (e.g.,between about 1 to 20, e.g., between about 1 to 15, e.g., between about1 to 10, e.g., between about 1 to 8), are preferred surfactants andprovide the most consistent nanowire dispersability in aqueous (ornon-aqueous) solution. The most preferable dispersants were found to bethose surfactants with both lower molecular weight and lower HLB values(e.g., Pluronic series surfactants Pluronic L-31, Pluronic L-62 andPluronic L-81; SilWet series surfactants SilWet L-77, SilWet L-7280, andSilWet L-7608; and Surfynol surfactants Surfynol 420 and Surfynol 440),and the combination of the surfactants with long chain polymericdispersants (e.g., Ammonium polyacrylate Acrysol G-111 (AG-111), PVA,and PVP) which provide good wire dispersity and surface property of thecoating. It was found that because of the typically high surface area(e.g., greater than above 28 cm²/g) and high aspect ratio (e.g.,typically greater than about 1,000) of the nanowires, the content of thedispersant(s) used to disperse the nanowires in aqueous solution shouldbe in a range of between about 0.01 to 20 wt %, more preferably in therange of about 1.0 to 15.0 wt %, for example, between about 1.0 and 8 wt% based on the total weight of the nanowire solution. Generally, if thecontent is less than 0.5 wt %, the dispersion of the nanowires tends tobecome insufficient and wire aggregation will occur. If the dispersantconcentration is too high, the nanowire dispersion composition containsthe dispersant in an amount that gives too high a viscosity, which cannegatively affect the ability to apply the nanowire dispersioncomposition to a device substrate. Furthermore, if the dispersantconcentration is so high as to exceed the solubility of the dispersantin the liquid medium, phase separation will occur. The appropriateamount of dispersant will depend on a number of factors such as the typeof nanowires to be dispersed, the size, shape, composition and aspectratio of the nanowires, the identity of the liquid medium, e.g., whetherthe liquid medium comprises a polar solvent such as water of a non-polarorganic solvent, etc.

To the nanowire dispersion composition of the present invention, one ormore binders may be added as necessary. The binder is preferably apolymeric or polymerisable binder, more preferably a water-soluble orwater-dissipatable polymeric or polymerisable binder. Examples ofwater-soluble binders include starches, e.g., hydroxy alkyl starches,for example hydroxyethylstarch; celluloses, for example cellulose,methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose,hydroxyethyl methyl cellulose and carboxymethlycellulose (and saltsthereof) and cellulose acetate butyrate; gelatin; gums, for exampleguar, xanthan gum and gum arabic; polyvinylalcohol; polyvinylphosphate;polyvinylpyrrolidone; polyvinylpyrrolidine; polyethylene glycol;hydrolysed polyvinylacetate; polyethylene imine; polyacrylamides, forexample polyacrylamide and poly(N,N-dimethyl acrylamide);acrylamide-acrylic acid copolymers; polyvinylpyridine;polyvinylphosphate; vinylpyrrolidone-vinyl acetate copolymers; vinylpyrrolidone-styrene copolymers; polyvinylamine; poly(vinylpyrrolidonedialkylaminoalkyl alkylacrylates), for example polyvinylpyrrolidone-diethylaminomethylmethacrylate; acid-functional acrylicpolymers and copolymers, for example poly(meth)acrylic acid andcopolymers of (meth)acrylic acid and other (meth)acrylate monomers;amine-functional acrylic polymers and copolymers, for examplepolydimethylaminoethylmethacrylate; acid or amine functional urethanepolymers, e.g., those containing dimethylolpropanoic acid and/or pendantor terminal polyethylene glycols; ionic polymers, especially cationicpolymers, for example poly (N,N-dimethyl-3,5-dimethylene piperidiniumchloride); polyesters, preferably those which carry water-solubilisinggroups, especially acid groups, for example polyesters obtainable bypolymerising a polyol with sodiosulphoisophthalic acid.

Examples of water-dissipatable binders are water-dissipatable polymers,for example, latex polymers, for example cationic, nonionic, and anionicsurface modified styrene-butadiene latexes; vinyl acetate-acryliccopolymer latexes; acrylic copolymer latexes which carry quaternaryammonium groups, for example a polymethylacrylate trimethylammoniumchloride latex; and dispersions of poly(acrylate), poly(methacrylate),polyester, polyurethane or vinyl polymers and copolymers thereof. Thepolymer dispersions may be prepared, for example, by emulsion,suspension, bulk or solution polymerisation followed by dispersion intowater. The binder may comprise a single binder or comprise a mixture oftwo or more binders, especially the exemplary binders described above.

Oligomeric polyols may be used to provide toughness and hydrophobic orhydrophilic characteristics to a polymer dispersant. Oligomeric polyolsare defined as polyols having a number average molecular weight betweenabout 500 and 5000 Daltons. Preferred members of this class arepolyester diols, polyether diols and polycarbonate diols.

Other useful additives which will also help to control drying rateinclude trimethylol propane, urea and its derivatives, amides,hydroxyether derivatives such as butyl carbitol or Cellosolve™, aminoalcohols, and other water soluble or water miscible materials, as wellas mixtures thereof. Other additives commonly known in the art includebiocides, fungicides, defoamers, corrosion inhibitors, viscositymodifiers, pH buffers, penetrants, sequestering agents, and the like.The nanowires may also be incorporated with a water-soluble high polymersuch as PVA or PVP, a thermosetting resin such as acryl emulsion, or acrosslinking agent such as ADC or diazonium salt may be added, ifnecessary. Heating of the nanowire dispersion composition may also helpthe surface uniformity of coatings.

Oxidation of Nanowires to Improve their Dispensability andProcessability

In order to further improve the dispersability and processability ofnanowires (e.g., inorganic nanowires such as semiconductor (e.g.,silicon) nanowires) in solvents such as aqueous solutions and make iteasier for nanowire thin film deposition, the nanowire surface may befurther treated by oxidation (e.g., either on or off of the growthsubstrate) prior to dispersing them in the solvent. The surfaceproperties of naturally grown nanowires tend to be quite complicatedbecause the wire surfaces are often only partially oxidized with anamorphous oxide layer during the nanowire synthesis process describedpreviously. For example, it has been found that as-grown siliconnanowires contain multifunctional surface groups with differentdistributions, charges and/or surface properties. These types of wirestend to be more difficult to disperse in aqueous media even in thepresence of one or more dispersants or surfactants as describedpreviously. The oxidation of inorganic nanowires, for example, rendersthe surface of the nanowires more uniform, consequently allowing thewires to more easily be suspended in solution with less dispersant andwithout agglomeration.

The nanowires may be oxidized using a variety of processes includingthermal oxidation or chemical oxidation (e.g., by chemically modifyingthe surfaces of nanowires in solution by, for example, using smallmolecules such as silanes with different end groups). In one embodiment,the nanowires are rapidly thermally oxidized by placing the nanowirebearing wafers into a rapid thermal oxidation (RTO) chamber whichtypically ramps the temperature of the chamber up to about 500° C. in aN2 atmosphere. In the RTO chamber, the temperature is then elevatedrapidly, e.g., 100° C./sec, to the desired temperature, e.g., >850° C.(typically between 900 and 1100° C.) in oxygen, and allowed to sit in O2saturated atmosphere for several minutes. The temperature is thenbrought back down to ambient temperatures in N2. Typically, attemperatures between 900 and 1100° C. for 1 to 10 minutes yields oxidelayers of from about 50 to about 100 angstroms. Similar processes may beemployed to provide a nitride or oxynitride shell on the core nanowire,using different reactive gases in addition to O2, including, e.g., NH3,N2O or NO. Alternatively, nitridation can be done over the oxide layer,e.g., by introducing such gases into the RTO chamber after oxidation.Likewise, RTO processes can be combined with alternating etch steps in a“grow-etch-grow” process, in order to provide a relatively defect freeinterface between the core nanowire and its surrounding oxide layer, byremoving surface contamination and defects in the silicon nanowire (see,e.g., U.S. Pat. No. 6,380,103). While the etching step may be performedwithin the RTO chamber, it is generally less desirable as it may impactoxide formation steps. Typically, a vapor etch step is performed beforeoxide formation. Alternatively to using a RTO chamber, the nanowires maybe oxidized in a conventional oxidation furnace using standard oxidationprocesses.

Applications of Nanowire Dispersion Compositions

The present invention also provides for electronic circuits comprisingthe nanowire dispersion compositions of the present invention. Suitablystable nanowire suspensions using the teachings of the present inventionare useful building blocks for high performance electronics. Acollection of nanowires orientated in substantially the same directionwill have a high mobility value. Furthermore, nanowires can be flexiblyprocessed in solution to allow for inexpensive manufacture. Collectionsof nanowires can be easily assembled onto any type of substrate fromsolution to achieve a thin film of nanowires. For example a thin film ofnanowires used in a semiconductor device can be formed to include 2, 5,10, 100, and any other number of nanowires between or greater than theseamounts, for use in high performance electronics.

The nanowires of the present invention can also be used to make highperformance composite materials when combined with polymers/materialssuch as organic semiconductor materials, which can be flexibly spin-caston any type of substrate. Nanowire/polymer composites can provideproperties superior to a pure polymer materials. Further detail onnanowire/polymer composites is provided below.

Collections or thin films of nanowires of the present invention can bealigned into being substantially parallel to each other, or can be leftnon-aligned or random. Non-aligned collections or thin films ofnanowires provide electronic properties comparable or superior topolysilicon materials, which typically have mobility values in the rangeof 1-10 cm²/V·s.

Aligned thin films of nanowires of the present invention can be obtainedin a variety of ways. For example, aligned thin films of nanowires canbe produced by using the following techniques: (a) Langmuir-Blodgettfilm alignment; (b) fluidic flow approaches, such as described in U.S.Pat. No. 6,872,645, and incorporated herein by reference in itsentirety; and (c) application of mechanical shear force. For example,mechanical shear force can be used by placing the nanowires betweenfirst and second surfaces, and then moving the first and second surfacesin opposite directions to align the nanowires. Aligned thin films ofnanowires/polymer composites can be obtained using these techniques,followed by a spin-casting of the desired polymer onto the created thinfilm of nanowires. For example, nanowires can be deposited in a liquidpolymer solution, alignment can then be performed according to one ofthese or other alignment processes, and the aligned nanowires can thenbe cured (e.g., UV cured, crosslinked, etc.). An aligned thin film ofnanowires/polymer composite can also be obtained by mechanicallystretching a randomly oriented thin film of nanowires/polymer composite.

P-doped nanowires and n-doped nanowires can be separately fabricated,and deposited in a homogeneous mixture onto a surface, such as amacroelectronic substrate. On a macroscopic level, the resultingmaterial appears to contain a high concentration of both n- andp-dopants. By creating such a mixture of p- and n-doped nanowires,macroelectronic devices can be fabricated that respond as if they areboth n- and p-doped. For example, a resulting thin film of nanowiresthat includes both n-doped and p-doped nanowires can exhibitcharacteristics of both n-doped and p-doped nanowires. For example,diode, transistor, and other known electrical devices can be fabricatedto include a combination of p-doped nanowires and n-doped nanowires.

Nanowire dispersion compositions of the present invention can also beused to produce electrical devices such as p-n diodes, transistors, andother electrical device types, using nanowire heterostructures asdescribed herein. Nanowire heterostructures include a plurality of p-njunctions along the length of the nanowire or in one or multiplecore-shell structure(s) and can include alternating portions or segmentsalong their lengths and/or coaxially that are differently doped.

Numerous electronic devices and systems can incorporate semiconductor orother type devices with thin films of nanowires produced by theprocesses of the present invention. Some example applications for thepresent invention are described below or elsewhere herein forillustrative purposes, and are not limiting. The applications describedherein can include aligned or non-aligned thin films of nanowires, andcan include composite or non-composite thin films of nanowires.

Semiconductor devices (or other type devices) can be coupled to signalsof other electronic circuits, and/or can be integrated with otherelectronic circuits. Semiconductor devices can be formed on largeflexible or rigid substrates, which can be subsequently separated ordiced into smaller substrates. Furthermore, on large substrates (i.e.,substrates substantially larger than conventional semiconductor wafers),semiconductor devices formed thereon can be interconnected.

The nanowire dispersion compositions of the present invention can alsobe incorporated in applications requiring a single semiconductor device,and to multiple semiconductor devices. For example, the nanowiredispersion compositions of the present invention are particularlyapplicable to large area, macro electronic substrates on which aplurality of semiconductor devices are formed. Such electronic devicescan include display driving circuits for active matrix liquid crystaldisplays (LCDs), organic LED displays, field emission displays. Otheractive displays can be formed from a nanowire-polymer, quantumdots-polymer composite (the composite can function both as the emitterand active driving matrix). The nanowire dispersion compositions of thepresent invention are also applicable to smart libraries, credit cards,large area array sensors, and radio-frequency identification (RFID)tags, including smart cards, smart inventory tags, and the like.

The nanowire dispersion compositions of the present invention are alsoapplicable to digital and analog circuit applications. In particular,the nanowire dispersion compositions of the present invention are usefulin applications that require ultra large-scale integration on a largearea substrate. For example, a thin film of nanowires produced by theprocesses of the present invention can be implemented in logic circuits,memory circuits, processors, amplifiers, and other digital and analogcircuits.

The nanowire dispersion compositions of the present invention can beapplied to photovoltaic applications. In such applications, a clearconducting substrate is used to enhance the photovoltaic properties ofthe particular photovoltaic device. For example, such a clear conductingsubstrate can be used as a flexible, large-area replacement for indiumtin oxide (ITO) or the like. A substrate can be coated with a thin filmof nanowires that is formed to have a large bandgap, i.e., greater thanvisible light so that it would be non-absorbing, but would be formed tohave either the HOMO or LUMO bands aligned with the active material of aphotovoltaic device that would be formed on top of it. Clear conductorscan be located on two sides of the absorbing photovoltaic material tocarry away current from the photovoltaic device. Two different nanowirematerials can be chosen, one having the HOMO aligned with that of thephotovoltaic material HOMO band, and the other having the LUMO alignedwith the LUMO band of the photovoltaic material. The bandgaps of the twonanowires materials can be chosen to be much larger than that of thephotovoltaic material. The nanowires, according to this embodiment, canbe lightly doped to decrease the resistance of the thin films ofnanowires, while permitting the substrate to remain mostlynon-absorbing.

Hence, a wide range of military and consumer goods can incorporate thenanowires dispersion compositions of the present invention. For example,such goods can include personal computers, workstations, servers,networking devices, handheld electronic devices such as PDAs and palmpilots, telephones (e.g., cellular and standard), radios, televisions,electronic games and game systems, displays (e.g., active matrixdisplays), home security systems, automobiles, aircraft, boats, medicaldevices, other household and commercial appliances, and the like.

For the myriad electronics applications described above, thesurfactants, binders, stabilizers and/or other chemicals in the nanowiredispersion composition that have been added to a suspension of nanowiresto improve coating properties may have a negative impact on theelectrical properties of the thin film formed by the nanowires. In thesecases, it may be desirable to remove these chemicals without disturbingthe nanowires, e.g., to provide adequate electrical connectivity betweenthe nanowire surface and the device contacts following deposition of thenanowires on the substrate surface. Some methods, such as washing theresidual chemicals away, tend to remove the nanowires as well. Inaddition to chemical methods of cleaning, physical methods such asplasma cleaning using, e.g., an oxygen, radio frequency plasma to removesurface contaminants can be employed. Other physical methods which canbe used to clean the substrate surfaces include laser cleaning, otherlow damage, gas phase cleaning methods, and perhaps molecular cleaning(e.g., the removal of all contamination on a molecular scale).

EXAMPLES

The following non-limiting examples illustrate nanowire dispersioncompositions according to the teachings of the present invention.

Materials: The following raw materials were used to prepare the nanowiredispersion compositions in the Examples which follow:

The nonionic surfactants used in the Examples are Surfynol 420, Surfynol440, Surfynol 211, and Surfynol 221 obtained from Air products, PA; theblock copolymer of Pluronic-17R, Pluronic L-31, Pluronic L-61, PluronicL-62, Pluronic L-64, Pluronic L-81, Pluronic L-92, Pluronic L-101,Pluronic F-77, and Pluronic F-88 were obtained from BASF; SilwetL-77,SilwetL-7608, SilwetL-7087, SilwetL-7280, and SilwetL-8620 were obtainedfrom GE Silicones.

The ammonium polyacrylate was commercially obtained as ACRYSOL G-111from Rhom & Hass. Polyurethane latex designated as SANCORE 2725 wasobtained from Noveon, Inc. Cleveland, Ohio. Polyvinyl alcohol (PVA),polyvinyl pyrrolidone (PVP), Triton X-100, Triton X-405, Tween 40, andSpan 20 were obtained from Aldrich.

Table 1 provided below shows the various anionic, cationic, nonionic andpolymeric surfactants which were used in the supporting Examples below.

TABLE 1 ID MW HLB Dispersants Inf. Venders Surfynol 440 NA 8 Nonionicsurfactant Air Products and Chemicals, inc. Surfynol 420 NA 4 Nonionicsurfactant Air Products and Chemicals, inc. Surfynol 211 NA 8-11Nonionic surfactant Air Products and Chemicals, inc. Surfynol 221 NA11-15 Nonionic surfactant Air Products and Chemicals, inc. PluronicL-17R4 2650 12 Block copolymer BASF Corporation Pluronic L-64 2900 15Block copolymer BASF Corporation Pluronic L-62 2500 7 Block copolymerBASF Corporation Pluronic L-81 2750 2 Block copolymer BASF CorporationPluronic F-77 6600 25 Block copolymer BASF Corporation Pluronic F-8811400 28 Block copolymer BASF Corporation Pluronic L-101 3800 1 Blockcopolymer BASF Corporation Pluronic L-61 2900 15 Block copolymer BASFCorporation Pluronic L-31 1100 5 Block copolymer BASF CorporationPluronic L-92 3650 6 Block copolymer BASF Corporation SilWet L-77 6005-8 Trisiloxane, all EO Crompton Corp. SilWet L-7280 600 5-8Trisiloxane, EO/PO (60/40) Crompton Corp. SilWet L-7087 20000  9-12Pendant siloxane, EO/PO Crompton Corp. (40/60) SilWet L-7608 600 5-8Trisiloxane, all EO Crompton Corp. SilWet L-7650 3000 5-8 Pendantsiloxane, all EO Crompton Corp. SilWet L-8620 2000 5-8 Linear siloxane,all EO Crompton Corp. Triton-X100 ~660 13.5 Polyoxyethylene(10) Aldrichisooctylcyclohexyl ether Triton-X405 ~2000 17.9 Polyoxyethylene(40)Aldrich isooctyclphenyl ether Span 20 346 8.6 Nonionic surfactantAldrich Tween 40 1248 15.6 Nonionic surfactant Aldrich SANCORE 2725 NAPolyurethane latex Noveon, Inc Acrysol G-111 NA Ammonium PolyacrylateROHM&HAAS PVA ~40,000 Polyvinyl alcohol Aldrich PVP 55,000 Polyvinylpyrrolindone Aldrich

Nanowire synthesis: Crystalline Si nanowires were grown using the VLSmethod described above. Wires were grown at atmospheric pressure onfour-inch wafers using SiH₄ as the gas precursor. The nanowire growthoccurred at 490° C. for 90 minutes. Au nanoparticles of 60 nm indiameter were randomly placed on the wafers and used as catalyst for theVLS reaction. All wafers were processed in the same way.

Sonication: Every four nanowire growth wafers were separately immersedin a 1000 ml beaker with 150 ml de-ionized (DI) water/ethanol (4/1)solution. The beaker was sonicated in a water bath to transfer nanowiresinto the solution by using power level 5 for 2 min. The nanowiresolution was concentrated by stirred cell membrane filtration.

Stirred cell membrane filtration: The nanowire solution was concentratedby using a 43 mm diameter stirred cell. A polycarbonate membrane with an8 um pore size was used to filter out the water as well as some smallwire junks (<10 um) and keep the concentrated nanowires at the top ofthe membrane.

Control 1

The 0.01 weight percent nanowire in DI water solution was coated on aVITEX film by using a #8 Meyer rod. The coating was dried in an 80° C.oven for 5 min. Then, a dark field optical microscope was used to recordthe images of the nanowire coated film. The wire dispersability wasvisually inspected based on the level of the wire aggregates on thepicture. It was found that there were many large wire aggregates in thissample and the wire dispersion was very poor.

Example 1-1

A dispersion mixture was made by adding 0.04 g of Surfynol 440 to 2 g ofnanowire solution (in Control 1) and mixing. The mixture was coated on aVITEX film surface by using a #8 Meyer rod. The coating was dried in an80° C. oven for 5 min. Then, an optical microscope was used to recordthe wire dispersion on the coating. The wire dispersability was visuallyinspected based on the level of the wire aggregates. It was found thatcompared to the Control 1 (without surfactant), much better wiredispersion of the coating was noticed in Example 1-1.

Example 1-2

Example 1-2 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.01 g of polyvinyl alcohol wasadded to 2 g nanowire solution (0.01 weight percent nanowire) and mixedtherewith. It was found that compared to the Control 1 (withoutsurfactant), better wire dispersion of the coating was noticed inExample 1-2.

Example 1-3

Example 1-3 was prepared according to the procedure for Example 1-1except that in addition 0.01 g of polyvinyl alcohol was added to themixture of Example 1-1. It was found that compared to Control 1 andExample 1-2, wire dispersion in Example 1-3 was further improved bymixing together both short chain surfactant (Surfynol 440) and longchain water soluble polymer (PVA).

Example 1-4

Example 1-4 was prepared according to the procedure for Example 1-3except that instead of polyvinyl alcohol, 0.03 g of polyvinylpyrrolidone (PVP) was added to the mixture of Example 1-3. It was foundthat compared to Control 1 and Examples 1-2, the addition of both shortchain surfactant (Surfynol 440) and long chain water soluble polymer(PVP) provided further improvement in wire dispersion.

Example 1-5

Example 1-5 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Surfynol 211 was added to2 g nanowire solution (0.01 weight percent nanowire) and mixedtherewith. It was found that compared to the Control 1 (withoutsurfactant), better wire dispersion of the coating was noticed inExample 1-5, although the coating uniformity was not as good as in theprevious Examples 1-1 through 1-4.

Example 1-6

Example 1-6 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Surfynol 221 was added to2 g nanowire solution (0.01 weight percent nanowire) and mixedtherewith. It was found that compared to the Control 1 (withoutsurfactant), better wire dispersion of the coating was noticed inExample 1-6.

Example 1-7

Example 1-7 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Triton X-100 was added to2 g nanowire solution (0.01 weight percent nanowire) and mixedtherewith. It was found that compared to the Control 1 (withoutsurfactant), better wire dispersion of the coating was noticed inExample 1-7.

Example 1-8

Example 1-8 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Triton X-405 was added to2 g nanowire solution (0.01 weight percent nanowire) and mixedtherewith. It was found that compared to the Control 1 (withoutsurfactant), better wire dispersion of the coating was noticed inExample 1-8.

Example 1-9

Example 1-9 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Span 20 was added to 2 gnanowire solution (0.01 weight percent nanowire) and mixed therewith. Itwas found that compared to the Control 1 (without surfactant), betterwire dispersion of the coating was noticed in Example 1-9.

Example 1-10

Example 1-10 was prepared according to the procedure for Example 1-1except that instead of Surfynol 440, 0.03 g of Tween 40 was added to 2 gnanowire solution (0.01 weight percent nanowire) and mixed therewith. Itwas found that compared to the Control 1 (without surfactant), betterwire dispersion of the coating was noticed in Example 1-10.

The results of the first set of Examples are summarized in Table 2below. As shown, the surfactants having lower molecular weights andlower HLB values (e.g., less than about 10, e.g., less than about 8)such as Surfynol 420 and 440 surfactants, provided the most improvednanowire dispersability compared to the Control. The proper combinationof one or more water soluble polymers such as PVP or PVA to thedispersion composition with surfactant can further improve thedispersability of the nanowires and the coating surface properties.

TABLE 2 Wt % Nanowire Nanowire (based on total dispersability onsolution Wt of Nanowires coated VITEX film Samples (g) Dispersantssolution) versus Control 1 Example 1-1 2 Surfynol 440 2 Improved Example1-2 2 PVA 1 Improved Example 1-3 2 Surfynol 440/PVA 2/0.5 Much ImprovedExample 1-4 2 Surfynol 440/PVP 2/1.5 Much Improved Example 1-5 2Surfynol 211 1.5 Improved Example 1-6 2 Surfynol 221 1.5 ImprovedExample 1-7 2 Triton X-100 1.5 Improved Example 1-8 2 Triton X-405 1.5Improved Example 1-9 2 Span 20 1.5 Improved Example 1-10 2 Tween 40 1.5Improved

Control 2

Control 2 was prepared according to the procedure for Control 1 exceptthat 0.05 weight percent nanowire in DI water/ethanol (3/7) solution wascoated on a VITEX film by using a #8 Meyer rod. The coating was dried inan 80° C. oven for 5 min. Then, a dark field optical microscope was usedto record the images of the nanowire coated film. The wiredispersability was visually inspected based on the level of the wireaggregates on the picture. It was found that there were many large wireaggregates in this sample and the wire dispersion was very poor.

Example 2-1

Example 2-1 was prepared according to the procedure for Control 2 exceptthat 0.05 g of Pluronic-17R4 was added to 1 g nanowire solution (Control2 nanowire solution) and mixed therewith. The optical microscope pictureresult shows that compared to the Control 2 (without surfactant), betterwire dispersion of the coating was noticed in Example 2-1.

Example 2-2

Example 2-2 was prepared according to the procedure for Example 2-1except that instead of Pluronic-17R4, 0.05 g of Pluronic L-64 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that compared toControl 2 (without surfactant) and Example 2-1, better wire dispersionof the coating was noticed in Example 2-2.

Example 2-3

Example 2-3 was prepared according to the procedure for Example 2-1except that instead of Pluronic-17R4, 0.05 g of Pluronic L-62 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that compared toControl 2 (without surfactant) and Examples 2-1 and 2-2, better wiredispersion of the coating was noticed in Example 2-3.

Example 2-4

Example 2-4 was prepared according to the procedure for Example 2-1except that instead of Pluronic-17R4, 0.05 g of Pluronic L-81 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that compared tothe Control and Examples 2-1 through 2-3, the best wire dispersion ofthe coating was noticed in Example 2-4.

Example 2-5

Example 2-5 was prepared according to the procedure for Example 2-4except that instead of Pluronic L-81, 0.05 g of Pluronic L-101 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that the wiredispersion was improved over the Control 2, but was not as good ascompared to Example 2-4.

Example 2-6

Example 2-6 was prepared according to the procedure for Example 2-5except that instead of Pluronic L-101, 0.05 g of Pluronic F-88 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that the wiredispersion was similar to the Control 2.

Example 2-7

Example 2-7 was prepared according to the procedure for Example 2-6except that instead of Pluronic F-88, 0.05 g of Pluronic F-77 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. The optical microscope picture result shows that the wiredispersion was similar to the Control 2.

Example 2-8

Example 2-8 was prepared according to the procedure for Example 2-7except that instead of Pluronic F-77, 0.05 g of Pluronic L-31 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. Although the wire dispersion was improved over Control 2, thewire dispersion was not as good as compared to Example 2-4.

Example 2-9

Example 2-9 was prepared according to the procedure for Example 2-8except that instead of Pluronic L-31, 0.05 g of Pluronic L-92 was addedto 1 g nanowire solution (Control 2 nanowire solution) and mixedtherewith. Although the wire dispersion was improved over Control 2, thewire dispersion was not as good as compared to Examples 2-4 and 2-8.

Example 2-10

Example 2-11 was prepared according to the procedure for Example 2-2except that in addition 2% (based on total weight of nanowire solution)of SANCORE 2725 (polyurethane latex dispersion) was added to 1 g ofnanowire solution (Control 2 nanowire solution) and mixed therewith. Theoptical microscope picture result shows that the wire dispersion wasimproved as compared to the Control 2 (and Example 2-2). In addition,coating surface tackiness was improved by adding PU latex.

The results of the second set of Examples are summarized in Table 3below. As shown, the Pluronic polymer surfactants having lower molecularweights and lower HLB values (e.g., less than about 10, e.g., less thanabout 8) such as Pluronic L-31, L-62, and L-81, provided the mostimproved nanowire dispersability compared to the Control. The additionof PU latex to the nanowire dispersion can help protect the nanowiredispersion coating surface without sacrificing the dispersability of thenanowires.

TABLE 3 Wt % Nanowire Nanowire (based on total dispersability onsolution Wt of nanowire coated VITEX film Samples (g) Dispersantssolution) compared to Control 2 Example 2-1 1 Pluronic L-17R4 2.5Improved Example 2-2 1 Pluronic L-64 2.5 Improved Example 2-3 1 PluronicL-62 2.5 Much Improved Example 2-4 1 Pluronic L-81 2.5 Most ImprovedExample 2-5 1 Pluronic L-101 2.5 Improved Example 2-6 1 Pluronic F-882.5 Same Example 2-7 1 Pluronic F-77 2.5 Same Example 2-8 1 PluronicL-31 2.5 Much Improved Example 2-9 1 Pluronic L-92 2.5 Improved Example2-10 1 Pluronic L-64/ 2.5/2 Improved Sancure 2725

Control 3

Control 3 was prepared according to the procedure for Control 1 exceptthat 0.05 weight percent nanowire in DI water/ethanol (8/2) solution wascoated on a VITEX film by using a #8 Meyer rod. The coating was dried inan 80° C. oven for 5 min. Then, a dark field optical microscope was usedto record the images of the nanowire coated film. The wiredispersability was visually inspected based on the level of the wireaggregates on the picture. It was found that there were many large wireaggregates in this sample and the wire dispersion was very poor.

Example 3-1

Example 3-1 was prepared according to the procedure for Control 3 exceptthat 0.01 g of Silwet L-77 was added to 1 g nanowire solution (Control 3nanowire solution) and mixed therewith. The optical microscope pictureresult shows that compared to the Control 3, slightly better wiredispersion of the coating was noticed in Example 3-1.

Example 3-2

Example 3-2 was prepared according to the procedure for Example 3-1except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-1 nanowire solution and mixed again. After coating, theoptical microscope picture result shows that compared to the Example3-1, much better wire dispersion of the coating was noticed in Example3-2.

Example 3-3

Example 3-3 was prepared according to the procedure for Control 3 exceptthat 0.01 g of Silwet L-7608 was added to 1 g nanowire solution (Control3 nanowire solution) and mixed therewith. The optical microscope pictureresult shows that compared to the Control 3, slightly better wiredispersion of the coating was noticed in Example 3-3.

Example 3-4

Example 3-4 was prepared according to the procedure for Example 3-3except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-3 nanowire solution and mixed again. After coating, theoptical microscope picture result shows that compared to Example 3-3,much better wire dispersion of the coating was noticed in Example 3-4.

Example 3-5

Example 3-5 was prepared according to the procedure for Control 3 exceptthat 0.01 g of Silwet L-7280 was added to 1 g nanowire solution (Control3 nanowire solution) and mixed therewith. The optical microscope pictureresult shows that the wire dispersion of the coating was improved overthe Control 3.

Example 3-6

Example 3-6 was prepared according to the procedure for Example 3-5except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-5 nanowire solution and mixed again. After coating, theoptical microscope picture result shows that compared to the Example3-5, much better wire dispersion of the coating was noticed in Example3-6.

Example 3-7

Example 3-7 was prepared according to the procedure for Control 3 exceptthat 0.01 g of Silwet L-7087 was added to 1 g nanowire solution (Control3 nanowire solution) and mixed therewith. The optical microscope pictureresult shows that the nanowire dispersion was not as good as Control 3.

Example 3-8

Example 3-8 was prepared according to the procedure for Example 3-7except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-7 nanowire solution and mixed again. After coating, theoptical microscope picture result shows that compared to the Example3-7, much better wire dispersion of the coating was noticed in Example3-8.

Example 3-9

Example 3-9 was prepared according to the procedure for Control 3 exceptthat 0.01 g of Silwet L-7650 was added to 1 g nanowire solution (Control3 nanowire solution) and mixed therewith. The optical microscope pictureresult shows the nanowire dispersion was not as good as Control 3.

Example 3-10

Example 3-10 was prepared according to the procedure for Example 3-9except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-9 nanowire solution and mixed again. After coating, theoptical microscope picture result show that compared to the Example 3-9,much better wire dispersion of the coating was noticed in Example 3-10.

Example 3-11

Example 3-11 was prepared according to the procedure for Control 3except that 0.01 g of Silwet L-8620 was added to 1 g nanowire solution(Control 3 nanowire solution) and mixed therewith. The opticalmicroscope picture result shows that the nanowire dispersion was not asgood as compared to Control 3.

Example 3-12

Example 3-12 was prepared according to the procedure for Example 3-11except that in addition 0.01 g of AG-111 was added to the mixture ofExample 3-11 nanowire solution and mixed again. After coating, theoptical microscope picture result shows that compared to the Example3-11, much better wire dispersion of the coating was noticed in Example3-12.

The results of the third set of Examples are summarized in Table 4below. As shown, the SilWet surfactants having lower molecular weightsand lower HLB values such as SilWet L-77, L-7608, L-7087 and L-7280provided the most improved nanowire dispersability compared to theControl. The addition of water soluble polymers such as AG-111 to thenanowire dispersion composition further enhanced the dispersion of thenanowires in the liquid composition.

TABLE 4 Wt % Nanowire Nanowire (based on total dispersability onsolution Wt of nanowire coated film Samples (g) Dispersants solution)versus Control 3 Example 3-1 1 SilWet L-77 1 Improved Example 3-2 1SilWet L-77/AG-111 1/1 Much Improved Example 3-3 1 SilWet L-7608 1Improved Example 3-4 1 SilWet L-7608/AG-111 1/1 Much Improved Example3-5 1 SilWet L-7280 1 Improved Example 3-6 1 SilWet L-7280/AG-111 1/1Much Improved Example 3-7 1 SilWet L-7087 1 Improved Example 3-8 1SilWet L-7087/AG-111 1/1 Much Improved Example 3-9 1 SilWet L-7650 1Same Example 3-10 1 SilWet L-7650/AG-111 1/1 Improved Example 3-11 1SilWet L-8620 1 Same Example 3-12 1 SilWet L-8620/AG-111 1/1 Improved

Example 4

The following non-limiting example illustrates improved nanowiredispersion by forming an oxide layer on the nanowires prior todispersing the wires in solution, according to the teachings of thepresent invention

Nanowire synthesis: Crystalline Si nanowires were grown using the VLSmethod described above. Wires were grown at atmospheric pressure onfour-inch wafers using SiH₄ as the gas precursor. The nanowire growthoccurred at 490° C. for 90 minutes. Au nanoparticles of 60 nm indiameter were randomly placed on the wafers and used as catalyst for theVLS reaction. All wafers were processed in the same way.

Nanowire oxidation: Wafers with nanowires were subject to dry thermaloxidation at 900° C. in O₂ ambient at atmospheric pressure in a RapidThermal Oxidation (RTO) system. The oxidation phase was 8 minutes long,while a one minute chamber conditioning at 500° C. was run immediatelyprior to that. This process typically grows 10-12 nm of SiO2 in the RTOsystem. A similar process can be performed using standard furnaceoxidation systems, adjusting the duration accordingly. Oxidation alsocan be run using higher temperatures, typically up to about 1000° C. Theoxidation time needs to be shortened accordingly in that case. Thequality of the oxide is typically not very different within thisinterval.

Sonication: Every four nanowire growth wafers were separately immergedin to a 1000 ml beaker with 150 ml DI water/ethanol (2/1) solution. Thebeaker was sonicated in a water bath to transfer nanowires into thesolution by using power level 5 for 2 min. The nanowire solutions wereconcentrated by stirred cell membrane filtration.

Stirred cell filtration: Nanowire solution was concentrated by using a43 mm in diameter stirred cell. A polycarbonate membrane with a 12 umpore size was used to filter out the water and keep the concentratednanowires at the top of the membrane.

Control 4

Nanowires without further surface oxidation were sonicated off andconcentrated to 0.03 wt % in a DI water/ethanol (80/20) solution bymembrane filtration. A dispersion mixture was made by first adding 0.03g of Surfynol 440 and 0.015 g of Pluronic-L64 to 1 g of nanowiresolution (0.03 wt %) and mixing. Then, 0.015 g of Acrysol G-111 wasintroduced in solids to the mixture and mixed again. The final mixturewas coated on a VITEX film surface by using a 7 mm doctor blade handdrawdown. The coating was dried in an 80° C. oven for 5 min. Then, adark field optical microscope was used to image the wire dispersion ofthe coating. The wire dispersability was visually inspected based on thelevel of the wire aggregates. It was found that there were many largeaggregates in this coating.

Example 4-1

Example 4-1 was prepared according to the same procedure for the Control4 except that the surface oxidized nanowires were used to replace thenanowires used in the Control 4. After coating, the optical microscopepictures shows that compared to the Control 4, the aggregates in thecoating have been significantly reduced due to the surface oxidationtreatment.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

All publications, patents and patent applications mentioned in thisspecification are indicative of the level of skill of those skilled inthe art to which this invention pertains, and are herein incorporated byreference to the same extent as if each individual publication, patentor patent application was specifically and individually indicated to beincorporated by reference.

1. A method of improving the dispersability of a plurality of inorganicnanowires in an aqueous or non-aqueous solution comprising introducing alow molecular weight dispersant into the aqueous or non-aqueoussolution.
 2. The method of claim 1, wherein the dispersant has an HLBvalue less than about
 15. 3. The method of claim 1, wherein thedispersant has an HLB value less than about
 10. 4. The method of claim1, wherein the dispersant has a molecular weight less than about 3,000.5. The method of claim 1, wherein the dispersant is selected from anonionic or polymeric surfactant.
 6. The method of claim 5, furthercomprising introducing into the aqueous or non-aqueous solution one ormore low chain polymeric dispersants selected from the group comprisingAmmonium Polyacrylate Acrysol G-111, PVA, and PVP.
 7. The method ofclaim 1, wherein the dispersant is selected from the group comprising anonionic SilWet surfactant, a Surfynol surfactant, a Pluronic polymericsurfactant, and combinations and mixtures thereof.
 8. The method ofclaim 7, wherein the dispersant comprises a Pluronic polymer surfactant.9. The method of claim 1, wherein the nanowires comprise siliconnanowires.
 10. The method of claim 1, wherein the nanowires have anaspect ratio greater than about
 1000. 11. A method of improving thedispersability of a plurality of inorganic nanowires in an aqueous ornon-aqueous solution comprising oxidizing the nanowires using anon-chemical oxidation process prior to dispersing the nanowires in thesolution.
 12. The method of claim 11, wherein oxidizing the nanowirescomprises performing a thermal oxidation process at or above 900° C. toform one or more shell layers of SiOx on the nanowires.
 13. The methodof claim 12, wherein the one or more shell layers comprises SiO₂. 14.The method of claim 13, wherein the plurality of nanowires comprisesilicon nanowires.